1. Field of the Invention
The present invention relates to elastomer rubbers for use in dynamic applications as a replacement for natural rubber parts, particularly in engine mount applications. More particularly, the present invention pertains to EPDM rubbers exhibiting tensile and tear strength comparable to natural rubber while maintaining excellent weather, ozone and thermal resistance for use in articles subjected to dynamic loading.
2. Discussion of the Art
Ethylene-xcex1-olefin-diene rubbers, particularly ethylene-propylene-diene (EPDM) rubbers, are excellent all-purpose rubbers that are useful in a wide variety of applications, including the manufacture of hoses, seals and weather strips. As used herein, rubber is defined to mean a material that is capable of recovering from large deformations quickly and forcibly and is essentially insoluble in solvents. EPDM rubber is a rubber made up of ethylene and propylene repeating units with a smaller amount of diene units to effect crosslinking of the polymer chains. Due in part to the substantial absence of unsaturation in the polymer backbone, EPDM rubbers exhibit superior oxidative and ozone resistance, weather resistance and heat resistance compared to conjugated diene rubbers. In addition, EPDM rubbers compare favorably in cost to other elastomers and maintain their properties across a broad range of filler concentrations.
Dynamic applications are those applications in which fabricated parts are subjected to repeated stress forces and dynamic loading. EPDM rubbers are known to exhibit relatively poor dynamic fatigue resistance, wear resistance and tensile strength and are therefore not generally used in those applications subjected to dynamic loading. These types of parts are instead manufactured using elastomers with superior dynamic mechanical properties such as natural rubber, styrene-butadiene rubber, polychloroprene and blends thereof.
While these polymers provide acceptable performance and exhibit good processability, it would be highly desirable to develop an EPDM rubber that exhibited sufficient dynamic mechanical endurance to allow it to be used in various dynamic applications. Toward this end, EPDM has been blended with other elastomers in an effort to develop a rubber with increased dynamic mechanical properties that retains the oxygen, ozone and heat resistance of EPDM while maintaining or reducing the cost of the final composition. These elastomers have included conjugated diene rubbers and polychloroprene. The effectiveness of these compounds is restricted by the fact that the proportion of EPDM that may be utilized is fairly limited in order to produce a compound with acceptable mechanical properties. In addition, the processing of such compounds is often troublesome and expensive.
For instance, the conditions necessary for acceptable curing of EPDM and other elastomers that may be used often conflict. The cure incompatibility of EPDM and highly unsaturated diene rubbers is demonstrated by the poor performance of the resulting composition in stress-strain tests. In fact, such compositions generally perform worse than either pure polymer. This poor performance is due in part to several factors. One cause of this incompatibility is the difference in vulcanization rates. Optimal vulcanization for one of the rubbers will generally lead to poor vulcanization of the other. This, combined with the preference of various accelerators for one polymer over the other, makes it difficult to achieve satisfactory vulcanization for both polymers. A second factor that contributes to poor vulcanization is the difficulty in achieving uniform dispersion among the two rubbers. Significantly different solubility parameters produce poor compatibility between rubbers, resulting in difficulty when attempting to mix such rubbers to a uniform dispersion. This produces an inhomogenous product with irregular and non-uniform properties. Traditional compatibilizers such as terpene resins and surface activated low molecular weight polymers have not been effective in mitigating this incompatibility.
In another approach, various additives have been mixed with EPDM in an effort to increase its tensile strength and fatigue resistance. Increasing the amount of reinforcing filler and peroxide has been shown to increase both hardness and modulus of the final rubber. However, the increase of filler has also been shown to correspondingly decrease the dynamic flex fatigue resistance of the resulting product. Furthermore, high levels of peroxide may decrease the tear strength of the final product. Various metal salts of acrylic acids as well as metal salts of xcex1-xcex2-unsaturated organic acids have also been added to EPDM in attempts to increase the wear resistance, tensile strength and modulus of the elastomer under dynamic loading conditions. All of these methods require additional expense or are at least relatively difficult to process. Therefore, a need remains for an EPDM rubber suitable for dynamic applications which exhibits superior tensile and tear strength while maintaining weather, heat, oxygen and ozone resistance as well as ease of processing and moderate cost.
The present invention provides ethylene-propylene-diene rubbers for use in dynamic applications, which exhibit a good balance between high mechanical strength and high dynamic fatigue strength. The rubbers display tear strength and other physical and dynamic properties comparable to similar compounds based on natural rubber while maintaining the excellent oxidative and thermal resistance of EPDM rubber.
Preferably, The EPDM rubbers of the present invention comprise a high molecular weight EPDM with a broad molecular weight distribution, about 20 to about 50 phr of a processing oil, about 10 to about 80 phr of a carbon black and a cure system comprising sulfur, tetramethylthiuram disulfide (TMTD), and 2-mercaptobenzothiazole (MBT).
In addition, the preferred EPDM rubbers may also contain conventional EPDM additives such as fillers, extenders, plasticizers, oils, waxes and pigments in such amounts that do not detract from the properties of the compound.
There are several characteristics associated with the preferred EPDM rubbers in accordance with the present invention. The EPDM rubbers should have a very high molecular weight, an ethylene content of about 65% to about 75%, which allows the possibility of crystallization under shear stress as a self-reinforcing effect like that of natural rubber, and broad molecular weight distribution to ease the incorporation of carbon black, which is important in achieving good tear strength.
The preferred vulcanized EPDM rubbers of the present invention are obtained by processing the composition and curing the rubber under time and temperature conditions necessary for optimum vulcanization.